The present invention relates to a heating device which is fed with microwave energy. The device can be used for example for jointing pieces of plastic material or plastic coated cardboard by welding or by drying and hardening of glue when sealing packages for example.
In order to obtain a good heat sealing joint, the heating must be even along the whole part of the heating object where the joint is to be located. In those cases where the heating is made under continuous transport of the material at a constant speed, it is not so necessary that the heating is even in each point along the whole length of the heating device. Because of the continuous movement, the parts that have passed the heating device receive the same total amount of heat, even though the heating varies along the heating zone. In case the feeding is stepwise and in those cases when the heating object is stationary relative to the heating device during the heating, it is however absolutely necessary that the heating device gives an even heating. If the joint is to be of high quality it is also necessary that the heated edges are forcefully pressed together. In an automatic high-speed production process there are high demands on fast heating and fast pressing together and cooling of the joint also may be necessary. Cooling is necessary in those cases, where plastic coated materials are joined. The plastic is melted and the joint is not strong until the plastic material is cooled to setting temperature.
The pressing together can be made in several ways. The heating device can be constructed to exert a pressure by itself at the same time as the heating is made. Alternatively the pressing together can be made at a later time.
When heat sealing packagings of paper or cardboard a method has been used up to now, for example, in which heat is transmitted by pressure of heat jaws. In this method the heat must be conducted through the material, which makes the process slow. The highest temperature will be on the surface of the material and not in the joint, and the surface temperature must be kept under a certain limit so that the material will not be damaged. In another method used when heating can be made during movement of a heating object along a transport track, the heating is made by hot air from for example a gas flame. In this method it is, however, difficult to prevent the product in the packaging from being damaged by the hot air. Furthermore there is a great risk of fire if the process is disturbed in some way. Experiments have also been made, in which high frequency welding has been used, particularly at frequencies of 13 or 27 MHz. At these relatively low frequencies it has been necessary to use high field intensities with risks for leakages or to accept a long process time.
At microwave heating, at a frequency of for example 2450MHz, the heating can be made approximately 100 times as fast as at the high frequency heating mentioned above when using the same field intensity. By using a wave guide provided with a slot, glue or plastic coating can be heated by microwave energy in a relatively simple way. It is a problem, however, to get an even heating over a distance long enough to be particularly usable. In every kind of wave guide the heating is uneven, which is caused by energy being absorbed by the heating object along the wave guide and also by standing waves occurring in the wave guide. Furthermore it is difficult to use cooling elements of metal in this heating device, as the cooling elements will influence the function of the wave guide. Cooling elements of ceramic or plastic material can be used, but the cooling effect of such cooling elements is often unsatisfactory due to the relatively low heat conducting ability of said materials.
The present invention relates to a heating device which, when fed by microwave energy, will produce an even heating in a longitudinal heating area when heating stationary as well as moving objects. According to the invention this is provided by an electric field which is produced in the heating device and constant over the whole heating zone.
The heating device can, if required, be provided with cooling elements of metal, which cool the joint effectively immediately after the feeding of microwave energy has been broken, and the joint can be under pressure until it is sufficiently cooled.